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P = V × I
Power = Voltage × Current
P = V² / R
Power = Voltage² / Resistance
P = I² × R
Power = Current² × Resistance
Wattage is the measure of electrical power, representing the rate at which electrical energy is consumed or produced. Measured in watts (W), it tells you how much work a device can do or how much energy it uses per second. The fundamental formula is P = V x I (power equals voltage times current), but power can also be calculated as P = V squared / R or P = I squared x R. Every electrical device -- from a tiny microcontroller consuming milliwatts to an industrial motor consuming kilowatts -- is rated by its wattage, making it the universal language for comparing energy consumption and sizing electrical infrastructure.
Check whether you know voltage and current (use P = V x I), voltage and resistance (use P = V squared / R), or current and resistance (use P = I squared x R). Device nameplates, datasheets, or multimeter readings provide these values.
Use volts, amperes, and ohms. Convert milliamps to amps (divide by 1,000), kilohms to ohms (multiply by 1,000). A 500 mA current at 12V gives 0.5 x 12 = 6W, not 500 x 12.
Multiply voltage by current for the most common case. For example, a 120V outlet supplying 10A to a heater: P = 120 x 10 = 1,200W (1.2 kW). To estimate electricity cost, convert to kilowatt-hours by multiplying kW by hours of use.
For AC devices with motors or transformers, true power (watts) = V x I x power factor. A motor drawing 10A at 120V with a 0.8 power factor uses 960W of real power, not 1,200W. The power factor is usually listed on the device nameplate.
Your electricity bill is based on kilowatt-hours. Knowing the wattage of each device lets you calculate running costs: a 1,500W space heater running 8 hours per day at $0.12/kWh costs $1.44/day or about $43/month.
A standard 15A, 120V circuit handles 1,800W maximum. Totaling the wattage of all devices on a circuit prevents overloads, tripped breakers, and potential fire hazards from overloaded wiring.
Sizing a solar panel system or backup generator requires knowing the total wattage of your loads. Add up all device wattages and account for peak surge power to select equipment that meets your needs.
Typical power consumption for common household and office appliances. Actual values vary by model and efficiency rating.
| Appliance | Typical Wattage | Daily kWh (est.) | Monthly Cost (est.) |
|---|---|---|---|
| LED Light Bulb | 10 W | 0.08 kWh | $0.29 |
| Laptop Computer | 50 W | 0.40 kWh | $1.44 |
| Desktop Computer | 200 W | 1.60 kWh | $5.76 |
| Television (55") | 100 W | 0.50 kWh | $1.80 |
| Refrigerator | 150 W | 1.50 kWh | $5.40 |
| Washing Machine | 500 W | 0.50 kWh | $1.80 |
| Microwave Oven | 1,200 W | 0.30 kWh | $1.08 |
| Hair Dryer | 1,800 W | 0.30 kWh | $1.08 |
| Space Heater | 1,500 W | 12.00 kWh | $43.20 |
| Central Air Conditioner | 3,500 W | 28.00 kWh | $100.80 |
| Electric Oven | 5,000 W | 5.00 kWh | $18.00 |
| EV Charger (Level 2) | 7,200 W | 28.80 kWh | $103.68 |
* Monthly cost estimated at $0.12/kWh based on typical daily usage hours.
Watts measure instantaneous power (the rate of energy use), while watt-hours measure energy consumed over time. A 100W light bulb running for 10 hours uses 1,000 Wh (1 kWh). Your electricity meter and bill track kilowatt-hours, not watts.
In DC circuits, watts and VA are identical. In AC circuits, VA is the apparent power (V x I), while watts is the real power (V x I x power factor). The difference matters for motors, transformers, and UPS systems where current and voltage are not perfectly in phase.
In the US, a standard 15A, 120V outlet can supply up to 1,800W, but the NEC recommends a maximum continuous load of 80% (1,440W). A 20A circuit can supply up to 2,400W (1,920W continuous). Exceeding these limits will trip the circuit breaker.
Rated wattage is the maximum the device can consume. Many devices, especially those with variable-speed motors or sleep modes, draw significantly less during normal operation. A 200W desktop computer might average only 80W during typical office tasks. Use a plug-in power meter for actual measurements.
Multiply the device wattage by hours of daily use, then divide by 1,000 to get kWh. Multiply kWh by your electricity rate. For example: 1,500W heater x 8 hours = 12,000 Wh = 12 kWh x $0.12/kWh = $1.44 per day, or about $43 per month.